[IMAPx] 'STARTTLS not supported' error ignored

When a user has setup the STARTTLS encryption method, but the server doesn't
support it, then an error should be shown to the user, instead of using
unsecure connection. There had been two bugs in the existing code which
prevented this error from being used and the failure properly reported.

This had been filled at:
https://bugzilla.redhat.com/show_bug.cgi?id=1334842